A fuel cell is being developed as a replacement of a fossil fuel that is not eco-friendly. Differently from a general secondary cell, the fuel cell is for directly converting an energy difference between before and after a reaction generated as hydrogen and oxygen are electrochemically reacted into electric energy without a fuel combustion (oxidation reaction) by supplying a fuel (hydrogen or hydrocarbon) to an anode and supplying oxygen to a cathode.
The conventional fuel cell system, as shown in FIG. 1, comprises: a fuel cell stack 10 where a plurality of anodes 11 and cathodes 12 are stacked under a state that an electrolyte membrane (not shown) are disposed therebetween for generating electric energy by an electrochemical reaction of hydrogen and oxygen; a fuel tank 30 for storing a fuel for a hydrogen decomposition in order to supply to the anode 11; an air supply portion 20 for oxygen-including air to the cathode 12; and a conduit 40 for connecting each component of the fuel cell system.
A fuel pump 31 for pumping a fuel stored in the fuel tank 30 is installed between the fuel tank 30 and the anode 11 of the fuel cell stack 10.
The air supply portion 20 includes: an air compressor 22 for supplying air in the atmosphere to the cathode 12 the fuel cell stack 10; an air filter 21 for filtering air supplied to the fuel cell stack 10; and a humidifier 24 for humidifying air supplied to the fuel cell stack 10. The humidifier 24 is provided with a water tank 23 for supplying moisture thereto.
As a fuel of the fuel cell system, KBH4, NaBH4, etc. for decomposing hydrogen are used. In case that NaBH4 is used as a fuel, NaOH or KOH, electrolyte aqueous solution is added thereto. An unexplained reference numeral 50 denotes a load.
A process for generating electric energy by supplying a fuel to the conventional fuel cell will be explained as follows.
As the fuel pump 31 is driven according to a control signal of a control unit (not shown), a fuel stored in the fuel tank 30 is pumped thereby to be supplied to the anode 11 of the fuel cell stack 10. As the air compressor 22 is operated, air filtered by the air filter 21 is humidified by passing through the humidifier 24 thus to be supplied to the cathode 12 of the fuel cell stack 10.
When a fuel and air are supplied to the fuel cell stack 10, an electrochemical oxidation of hydrogen is performed in the anode 11 and an electrochemical de-oxidation of oxygen is performed in the cathode 12 under a state the electrolyte membrane (not shown) is positioned therebetween. Herein, a generated electron is moved thus to generate electricity. The generated electricity is supplied to the load 50.
In case that a fuel is NaBH and NaOH of electrolyte aqueous solution, a reaction performed in the anode is expressed a following chemical formula.2H2O+NaBH4→NaBO2+4H2 
After a reaction, impurity such as NaBO shown in the above formula is necessarily generated The impurity exists as an aqueous solution state and is solidified at a conduit, or the impurity is precipitated in the fuel tank thus to prevent a flow of a fuel. Therefore, it is necessary to replace a fuel or to remove impurity before impurity is excessively generated.
Replacement time of the fuel tank has to be determined by checking a consumption amount of a fuel. When a fuel is replaced in advance, an excessive amount of fuel remains in the fuel tank thus to cause a fuel waste. Also, when a fuel is replaced too late, the fuel is completely consumed thus to cause a problem that the system is shut-down.
In the conventional art, replacement time of a fuel was determined by referring to a driving time of the system or an accumulated consumption power used in the load. However, since a consumption amount of a fuel and a generation amount of impurity are different according to a driving condition such as a load size, temperature, humidity, etc., it was impossible to check a precise replacement time of the fuel tank.